K.S. Niranjana and S. Viswanath ([email protected])
Ever since the British introduced tea (Camellia sinensis) as a plantation crop on the slopes of Western Ghats, southern India at the turn of the 19th century, tea growers have been searching for that ideal companion shade tree.
Western Ghats experiences high rainfall, although not regularly distributed throughout the year, and tea is mainly grown as a rainfed crop. The success of tea plantations depends on the adequate and uniform yearly distribution of rainfall and soil moisture. This is where shade trees come in.
Over the years, Silver oak (Grevillea robusta) has evolved into perhaps the most preferred shade tree species in tea plantations. Its unique leaves effectively filter light and provide enough shade during the dry months. Because of its deep roots, Silver oak does not compete for nutrients and moisture with the tea plants. Moreover, it can withstand pollarding, lopping and desuckering. It recovers and grows fast, is immune to common pests and diseases, and can serve as windbreaks.
Resource-sharing patterns of Silver oak trees in some tea plantations in Munnar, Idukki district, Kerala revealed several interesting insights.
Fig. 1. Drought adversely affects tea bushes in open conditions.
Resource-sharing of tea plants and Silver oak
In rainfed plantations, tea growers always worry about the loss of soil moisture during the dry season. The topography of Western Ghats limits the construction of sprinkler irrigation systems. Hence, tea growers rely on the shade trees to maintain soil moisture.
Preliminary studies found that the leaf litter of shade trees helped maintain soil moisture in tea plantations without competing with the tea plants during the dry months. It also lessened fluctuations in soil temperature which harmed the root growth of tea plants, increased relative humidity, and provided high organic matter. This organic matter compensated for the loss of nutrients in tea plantations.
Studies estimated that Silver oak replenished as much as 6-8 t/ha per year of excellent mulch (assuming a tree density of 140/ha), through leaf litter. Nutrients returned to the soil through leaf litter, annual loppings, stemflow and throughfall were estimated at 75 kg N, 4 kg P2O5, 40 kg K2O, 95 kg Ca, 40 kg Mg and 38 kg S per hectare per year. In addition, Silver oak contributes significant amounts of micronutrients, thereby reducing further the application of fertilizers in the tea plantations of Western Ghats.
Under shaded conditions, researchers in China also found that tea plants produced soft and dark green tea shoots that produced good-quality green tea. Studies also showed that shaded conditions not only favored increased production but also a high cholorophyll content of tea plant leaves because of the reduced leaf temperature. Pest outbreaks were also found to be lower in shaded tea plantations than in those that were in open fields.
Different root architectures
In designing agroforestry systems, the different components should complement each other in terms of nutrients and at the same time share other natural resources. Tea plants and Silver oak have different root patterns and architectures so there is minimal competition for nutrients. The deep roots of the Silver oak absorb nutrients found beyond the lower strata of the soil where the roots of the tea plants absorb nutrients.
Fig. 2. Tea bushes have more horizontally oriented roots.
Reducing the adverse impacts of shade trees
Shade trees need to be managed properly to maximize their benefits. If the humidity of shaded tea plantations remains unchecked and continues to be high, tea plants become susceptible to the blister blight disease caused by Exobasidium vexans. This is considered one of the most serious epidemics of tea plantations as it attacks the tender leaves of the tea plants. This disease can be controlled by adjusting the time of pollarding the Silver oak to be completed by June, before the first incidence of the blister blight infection. For tea plantations in the northeast monsoon zone areas, pruning should be completed before August.
Fig. 3. Roots of silver oak penetrate deep into the soil layers to absorb nutrients, without competing with tea plants.
Tea plantation managers have also observed that shaded tea plants tended to be less responsive to fertilizers. However, studies suggested that the intake of fertilizers could be improved through proper shade regulation of the Silver oak canopy. Silver oak is usually planted in tea estates in Munnar at a spacing of 6 × 6 m.
While they are growing, the tree stands are thinned by removing the alternate trees in the east-west direction prior to pruning the tea bushes and finally stabilizing the shade tree population at 140 trees/ha. Hence, it is necessary to pollard and regulate the top shade at periodic intervals to allow optimum light infiltration.
Pollarding the shade trees having 50 cm dbh is ideally carried out when the tree is more than 9 m high. Excess side branches between 7.5 m and 9 m from the tree base should be removed, leaving two to three tiers of lateral branches to maintain a clean bole below this level. Annual lopping of the pollarded Silver oak trees is also done by trimming the lateral branches and removing only the erect-growing suckers to retain at least 20% of the foliage, and thereby maximize the benefits of pollarding.
Carbon sequestration potential of the shade trees
Integrating shade trees in tea plantations is a low-cost but effective carbon sequestration strategy. It is estimated that a 20-year-old Silver oak shade tree can sequester up to 41.8 Mg/ha of carbon.
The life span of Silver oak is up to 40 years before it can be cut and replanted. With the current market price for Silver oak timber estimated at US$265/m3, an additional income of US$65 950 can be earned per hectare after 40 years.
Present timber transit rules are liberal when it comes to cutting, transporting and selling Silver oak in Western Ghats. This enables small farmers to earn income from their Silver oak trees to offset losses from the tea industry.
Planting Silver oak as shade trees in the tea plantations of Western Ghats is a near-perfect example of complementary resource sharing in shaded perennial agroforestry systems.
(The author is a scientist at the Tree Improvement and Propagation Division, Institute of Wood Science and Technology, 18th Cross, Malleswarm, Bangalore 560 003, Karnataka State, India.)
Fig. 4. Silver oak shade trees add as much as 6-8 t/ha per year of leaf litter.
Fig. 5. Regulating the shade of Silver oak through pollarding is essential in managing tea estates.
